Conventional blood analyzers have provided separate incubators for separate functions--the test elements that provide a potentiometric assay are incubated separately from those that provide a colorimetric assay. However, there has been a need to combine features and to simplify, whereby a single incubator can be used to incubate both colorimetric and potentiometric test elements. In such a case, each station of the incubator has to be capable of receiving either type of test element.
Such a single incubator construction has had the problem, prior to this invention, of not having an evaporation cover that could be used for either test element. Instead, the cover used on the colorimetric incubator was useful only for the colorimetric test element, and similarly the cover in the potentiometric incubator was useful only for the potentiometric test element. The reason was that the cover is intended to seal off from evaporation, the areas of the test element wetted by the sample. In commonly used test elements such as those sold under the trademark "Ektachem" by Eastman Kodak, the wetted areas occupy different parts of the test element geometry. Furthermore, a recess is formed in the cover for the potentiometric elements, in light of the fact that liquid tends to protrude from such an element. However, no such liquid protrudes from the colorimetric element, and instead the cover needs to seal flush against the latter, to insure a minimum evaporation loss. Such a flush, sealing fit is not appropriate for a potentiometric test element, and indeed tends to cause contact with the sample drop on the test element, thus contaminating that station of the incubator.
Camming means have been provided in some analyzers to raise the cover as a test element, such as a potentiometric element, is loaded, as shown, for example, in FIG. 4 of U.S. Pat. No. 4,298,571. However, such a construction requires the analyzer to have a separate, active element for camming the cover away, instead of providing for such means on the cover itself.
Thus, prior to this invention it has not been possible to provide a single incubator for both types of test elements, because there has been available no evaporation cap that would be effective for both types.